1. Technical Field
The present disclosure relates to DC offset, and more particularly, to an apparatus for canceling a DC offset.
2. Discussion of the Related Art
In a typical direct conversion receiver, a DC offset is generated due to self-mixing of a local oscillator. The DC offset may deteriorate linearity and the noise figure may deteriorate as well, thus lowering the overall performance of a system. Thus, DC offset cancellation (DCOC) is an important design issue.
FIG. 1 shows an example of a timing diagram of the DCOC in a GSM (Global System for Mobile Communications) type direct conversion receiver. In a time division multiple access (TDMA) such as a GSM receiver, a signal is typically input after passing through a DC offset removing process using a DCOC control signal, as shown in FIG. 1. The DCOC operation is performed through a process of sensing and storing a DC offset voltage. After the DCOC operation is completed, the DC offset voltage is maintained until the next time slot.
Meanwhile, when the DC offset is canceled through the DCOC operation, the time for performing the DCOC is on the order of several tens of microseconds. It is often difficult to apply a structure to completely cancel an input DC offset using a DC shielding capacitor to the GSM type direct conversion receiver in which a switching operation is repeatedly performed at a cycle of several hundreds of microseconds through several milliseconds. For this reason, the GSM type direct conversion receiver generally adopts a feedback method.
FIG. 2 is a block diagram of a DC offset canceling circuit using a feedback method. Referring to FIG. 2, the DC offset canceling circuit includes a main amplifier 20 and a sensing/storing unit 21 forming a feedback loop at an output terminal of the main amplifier 20. The sensing/storing unit 21 operates in response to a DCOC control signal.
A DC offset voltage output from the main amplifier 20 is removed by the sensing/storing unit 21 and the feedback loop for a particular time, for example, the time during which the DCOC is performed. The sensing/storing unit 21 senses the DC offset voltage output from the main amplifier 20 and stores the sensed voltage. In the technology shown in FIG. 2, the feedback circuit or the feedback loop is a noise source and the noise figure characteristic deteriorates as a feedback gain is increased to improve the DCOC performance.
FIG. 3A is a block diagram of a DC offset canceling circuit using a feedback method. Referring to FIG. 3A, the DC offset canceling circuit uses an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). The DC offset canceling circuit includes an ADC, a DAC, and a register.
The DC offset canceling circuit converts a DC offset voltage to a digital signal using the ADC and stores the digital signal in the register. The DC offset canceling circuit supplies appropriate current to the circuit according to the digital signal, for example, an offset voltage, stored in the register using the DAC, thus canceling the DC offset.
FIG. 3B is a block diagram of a DC offset canceling circuit using a feedback method. Referring to FIG. 3B, the DC offset canceling circuit includes a trans-conductance (Gm) circuit, a capacitor C, and a switch. Since the switch is closed while the DCOC is performed, the capacitor C stores the DC offset voltage. The stored voltage is converted to current by the trans-conductance (Gm) circuit and the converted current is applied to the circuit so that the DC offset is canceled. The capacitance of the capacitor C must be sufficient to maintain the corrected DC voltage until the next time slot.
The technologies discussed above and shown in FIGS. 3A and 3B cancel the DC offset by adding or subtracting current to or from the circuit through the DC offset canceling circuit. In this case, the input of current by the feedback circuit and the DC offset canceling circuit itself become noise sources. As the feedback gain is increased to improve the DCOC performance, the amount of current used for the correction increases so that the noise figure characteristic is further deteriorated.